article: Have you ever heard someone use the phrase "once in a blue moon"? People use this expression to describe something that they do not do very often. For example, someone might say that he tries to avoid eating sweets because they are unhealthy, but will eat chocolate "once in a blue moon". Or someone who does not usually like to go to the beach might say "I visit the shore once in a moon." While many people use this phrase, not everyone knows the meaning behind it.
  The first thing to know is that the moon itself is never really blue. This is just an expression. In fact, the phrase "blue moon" has to do with the shape of the moon, not the color.
  As the moon travels around the earth, it appears to change shape. We associate names with certain shapes of the moon. For example, when we can see a small part of the moon, it is called a crescent moon. A crescent is a shape that looks like the tip of a fingernail. When we cannot see the moon at all, it is called a new moon. When we can see the whole moon, it is called a full moon. Usually, there is only one full moon every month. Sometimes, however, there will be two full moons in one month. When this happens, the second full moon is called a "blue moon".
  Over the next 20 years, there will only be 15 blue moons. As you can see, a blue moon is a very rare event. _ has led people to use the expression "once in a blue moon" to other very rare events in their lives.
question: When does a blue moon happen in nature?
options: When the moon has a blue moon. When we cannot see the moon at all. When there are two full moons in one month. When we can only see a small part of the moon.